1. Field of the Invention
The present invention provides a pixel circuit of an active-matrix organic light-emitting diode, and more particularly, a pixel circuit capable of compensating property variations in poly-Si TFTs.
2. Description of the Prior Art
Compared to a cathode ray tube (CRT) monitor, a flat panel display (FPD) monitor has incomparable advantages, such as low power consumption, no radiation, small volume, etc., so that the FPD monitor has become a substitute for the CRT monitor. As FPD technology advances, prices of FPD monitors are reduced, and sizes of FPD monitors are increased, which make FPD monitors more popular. Therefore, light, fine, colorful, low-power FPD monitors are expected, and a device that can combine these advantages is the Organic Light-Emitting Diode (OLED) display.
The OLED combines many characteristics together, such as self emission, a wide viewing angle (over 165°), short response time (about 1 μs), high brightness (100-14000 cd/m2), high luminance efficiency (16-38 Im/W), low driving voltage (3-9V DC), thin panel (2 mm), simplified manufacturing, low cost, etc., and the OLED can be applied for large-size or flexible panels. The principle of an OLED is that after conducting a bias voltage, electrons and holes are passing through a hole transport layer, an electron transport layer and then combine in an organic light emitting material to form “excitons”. Energy of the excitons is released to the ground state, and the released energy creates luminance of the OLED with colors.
According to different driving methods, the OLED can be divided into two kinds, and one is a passive matrix OLED, or PM-OLED, and the other is an active matrix OLED, or AM-OLED. Please refer to FIG. 1 and FIG. 2. FIG. 1 illustrates a schematic diagram of a PM-OLED of a pixel, while FIG. 2 illustrates a schematic of an AM-OLED of a pixel. In comparison, the structure of the PM-OLED shown in FIG. 1 is simple, so the cost is low. However, the PM-OLED must be operated under highpulse-currents to reach the brightness appropriate for human eyes. Moreover, the brightness of the PM-OLED is directly proportional to the operating current, and the higher the operating current, the lower the circuit efficiency, the life, and the resolution of the PM-OLED. As a result, the PM-OLED is usually utilized for small sized products. On the other hand, although cost and complexity of the AM-OLED are higher than the PM-OLED (but still lower than a TFT-LCD), yet each pixel can store driving signals and can be operated independently and continuously. Also, circuit efficiency of the AM-OLED is higher, so the AM-OLED is utilized for products of large size, high resolution, and high information capacity. However, there are many factors affecting performance of a large size AM-OLED panel.
As those skilled in the art recognize, in FIG. 2, a current IOLED flowing through the OLED can be derived as:
      I    OLED    =            1      2        ⁢          μ      ·              C        OX            ·              W        L            ·                        (                                    V              GS                        -                          V              TH                                )                2            Therefore, the current IOLED is affected by the threshold voltage VTH of the polycrystalline silicon thin-film transistor, or poly-Si TFT, as shown in FIG. 2, so that the performance of pixels varies with time and can not reach uniform image. In order to improve the performance, the prior art provides various pixel circuits for compensating the variation in the poly-Si TFT.
In the prior art, pixel circuits of the AM-OLED can be classified into: current driving, digital driving, and voltage driving pixel circuits. A current driving pixel circuit provides excellent image quality, but its panel driving speed is too slow to implement high resolution displays. A digital driving pixel circuit can reduce the poly-Si TFT threshold voltage variation sensitivity, but it needs a very fast addressing speed, so that it is not a good solution for high gray scale displays. A voltage driving pixel circuit can compensate the variation of threshold and is more attractive to integrate poly-Si TFT data drivers on a display panel. However, the prior art voltage driving pixel circuit still has some disadvantages.
For example, please refer to FIG. 3, which illustrates a prior art pixel circuit 30 of an AM-OLED. The pixel circuit 30 comprises an OLED 300, switching transistors 302, 304, 306, a driving transistor 308, capacitors 310, 312, scan-line signal reception ends 316, 318, 320, and a data-line signal reception end 314. The switching transistors 302, 304, 306, and the driving transistor 308 are poly-Si TFTs. The scan-line signal reception ends 316 and 320 receive first scan-line signal for controlling the switching transistors 302 and 306. The scan-line signal reception end 318 receives second scan-line signal for controlling the switching transistor 304. The data-line signal reception end 314 receives data-line signal (Vin) for driving the driving transistor 308 to output current IOLED to the OLED 300 and emit light at specific durations. In addition, according to characteristics of the OLED 300, the OLED 300 can be considered to be a transistor and a capacitor as an equivalent circuit 400 shown in FIG. 4. The equivalent circuit 400 includes a transistor 402 and a capacitor 404. A gate of the transistor 402 is coupled to a drain of the transistor 402, and the capacitor 404 is coupled between the drain and a source of the transistor 402.
Please refer to FIG. 5, which illustrates a time sequential signal waveform of the data line, the first scan line, and the second scan line. In FIG. 5, durations T1, T2, and T3 are an initialization period, a compensation period, and a data-input period respectively. Referring to FIG. 3 and FIG. 5, in the duration T1, the data-line signal are at a low voltage level, and the first scan-line signal and the second scan-line signal are at a high voltage level, so the switching transistors 302, 304, 306 are turned on. Then, electrons stored in a gate G and a source S of the driving transistor 308 flow through the switching transistors 302, 304, and 306 to the data-line signal reception end 314. Next, in the duration T2, the first scan-line signal stay at the high voltage level, the second scan-line signal change to the low voltage level, and the data-line signal change to the high voltage level, so the switching transistor 304 is tuned off. Then, the data-line signal is input to the gate G of the driving transistor 308 through the switching transistor 302. Since the data-line signal is at the high voltage level (Vin) in this case, a current flow generated from the drain D to the source S of the driving transistor 308 to the OLED 300. Meanwhile, the high-level data-line signal charges the capacitor 312, so that the capacitor 312 stores a voltage drop ΔV:
            Δ      ⁢                          ⁢      V        =                            a                      1            +            a                          ×                  V                      T            ⁢                                                  ⁢            H            ⁢                                                  ⁢            _            ⁢                                                  ⁢                          T              DV                                          -                        1                      1            +            a                          ×                  V                      TH            ⁢                                                  ⁢            _            ⁢                                                  ⁢            OLED                              +                        1                      1            +            a                          ×                  V                      i            ⁢                                                  ⁢            n                                    where    ,                  ⁢          a      =                                    K                          T              DV                                            K                          T              OLED                                            ,  KTDV and KTOLED are conduction parameters of the driving transistor 308 and the OLED 300 respectively,VTH—TDV and VTH—OLED are threshold voltages of the driving transistor 308 and the OLED 300 respectively.Next, in the duration T3, the data-line signal stay at the high voltage level, the first scan-line signal change to the low voltage level, and the second scan-line signal change to the high voltage level, so the driving transistor 308 stays on, the switching transistors 302, and 306 are turned off, and the switching transistor 304 is turned on. Therefore, data-line signals (Vin) charge the capacitor 312 through the switching transistor 304, and the gate voltage of the driving transistor 308 becomes Vin+ΔV. If an output (source) voltage of the driving transistor 308 is Vout, then a current IOLED flowing into the OLED 300 is:IOLED=KTDV·(VGS−VTH—TDV)2=KTDV·(Vin+ΔV−Vout−VTH—TDV)2 Therefore, the current flowing into the OLED 300 is changed with the voltage drop ΔV stored in the capacitor 312, where the voltage drop ΔV is varied with the threshold voltage. As a result, the current flowing into the OLED 300 is varied unexpectedly, causing non-uniformity of images between pixels and degradation of display quality.
In short, during the compensation period, the prior art pixel circuit 30 provides an unnecessary current to the OLED 300, and during the data-input period, the current flowing into the OLED 300 is affected by the threshold voltage, causing a bad gray level, a low contrast, and an increasing power consumption of the display panel.